(CNN) --The lead investigator into the space shuttle Columbia accident told congressional leaders Thursday that his task force "determined NASA is not a learning organization. They do not learn from their mistakes."

Retired Adm. Hal Gehman's harsh comments before a House committee came one day after a panel of Senators questioned him about the the Columbia Accident Investigation Board's final report. The CAIB, which Gehman chairs, released the 248-document one week ago.

The report lays out the direct cause of the February 1 accident that killed seven astronauts -- foam hitting the leading edge of the left wing -- and goes into to detail about the contributing factors leading up to the disaster.

Congress is looking for answers to the question of how to fix NASA.

"We were wrong twice, we can't afford to be wrong again." Rep. Ralph Hall, D-Texas, said in reference to the Challenger accident of 1986 and the Columbia disaster.

But Gehman insisted that the space shuttle is not inherently unsafe. Instead, he said, it is NASA's management process that is unsafe.

As an example, he said top shuttle managers told investigators there were no time pressures to meet schedules, while lower level engineers felt tremendous pressure to keep on schedule.

Rep. Dana Rohrabacher, R-California, asked Gehman: "Is it the recommendation of CAIB that if the international space station can be supplied by alternate systems that it should be?"

Gehman replied emphatically, "Separate the cargo from the people as soon as possible."

The priority should be a system to put people into space and return them to Earth as safely as possible, he said. Any other requirements, like reusability to reduce costs, the ability to also carry cargo, or additional functions besides crew transport, would eat into the vehicle's safety margin.

Even before the Columbia accident, plans for an "orbital space plane" to transfer astronauts to and from the space station were in the works.

But Gehman noted NASA's difficulties in getting Congress and the White House to agree on the purpose of the new plane.

"It would be a wonderful leap forward if you agreed the requirement was to develop a vehicle to go into and out of orbit safely," he said.

Gehman did not advise stopping shuttle flights permanently because the shuttle can be operated safely and is needed to complete the assembly of the orbiting outpost, he said.

NASA plans to release on Monday its plan for returning the remaining space shuttles to space. It is expected to address all of the CAIB's mandatory recommendations for changes that would need to be made before that could happen.